A positive displacement pump causes a fluid to move by trapping a fixed amount of the fluid and then forcing or displacing the trapped volume through a discharge outlet. Positive displacement rotary pumps are pumps that move fluid using the principles of rotation. At the inlet to the pump, the rotation captures and draws in the fluid before it is trapped and passed through the outlet. Various types of rotary pumps are available, including, but not limited to internal and external gear pumps, screw pumps, flexible vane or sliding vane pumps, liquid ring vacuum pumps, circumferential piston pumps, rotary lobe pumps etc. While this disclosure uses rotary lobe pumps and circumferential piston pumps as primary examples, one skilled in the art will realize that the principles disclosed herein are applicable to other types of rotary pumps as well.
Rotary lobe pumps (RLPs) are used in a variety of industries including, pulp and paper, chemical, food, beverage, pharmaceutical, and biotechnology. They are popular in these diverse industries because they offer sanitary qualities, high efficiency, reliability, corrosion resistance, and good clean-in-place and sterilize-in-place (CIP/SIP) characteristics.
RLPs offer a variety of rotor options including single, bi-wing, tri-lobe and multi-lobe rotors and lobes of different shapes. While RLPs are similar to external gear pumps in operation because both pumps employ two rotors and fluid flows around the interior of the casing. Unlike the gears of an external gear pump rotor, the lobes of the RLP rotor do not make contact. Lobe contact is prevented by external timing gears located in the gearbox.
As the two rotors of an RLP rotate, the lobes come in and out of mesh. As the lobes come out of mesh near the inlet port, they create expanding volume on the inlet side of the pump. Material flows into the cavity and is trapped by the lobes as they rotate. Pumped material travels around the interior of the casing in the pockets between the lobes and the casing. Finally, near the outlet port, the lobes go back into mesh, which forces material through the outlet port under pressure.
The gentle pumping action provided by the non-contacting lobes minimizes product degradation. RLPs also have large pumping chambers, allowing them to handle solids without damaging the solids. RLPs are used to handle slurries, pastes, and a wide variety of other liquids. If wetted, RLPs are self-priming RLPs also offer reversible flows and can operate dry for long periods of time. Flow is relatively independent of changes in process pressure, so output is constant and continuous.
Like an RLP, a circumferential piston pump (CPP) also has two rotors that are timed like rotary lobe pumps. A primary difference between a CPP and an RLP lies in the rotors and the casing. In a CPP, the rotors include wings (referred to as “pistons”) that rotate in annular or cylindrical chambers (or simply, “cylinders”) machined into the pump casing. This provides a large sealing surface between the pistons and the cylinders which minimizes slip. Because, CPPs have only two moving parts within the fluid chamber like RLPs, they have proven reliable. There is no sealing contact between the piston surfaces, which, like an RLP, distinguishes CPPs from gear and screw pumps; however, the CPP pistons make sealing contact with the cylinders machined into the pump housing. Similar to an RLP, external timing gears synchronize the movement of the rotors.
Like an RLP, as the CPP rotors rotate on the inlet side, the rotor pistons come out of mesh creating an expanding volume that draws the liquid into the pump. The liquid is forced out the outlet port by the collapsing cavity on the outlet side caused by the rotor pistons going back into mesh.
One disadvantage common to RLPs, CPPs and other rotary pumps is overheating of the shaft seals when pumping viscous fluids, leading to excessive seal wear and premature seal failure. Accordingly, rotary pump designs that optimize cooling and maximize seal life are needed to overcome this problem.